This invention relates to a projection optical device for projecting, through an optical system, a pattern formed on one object onto another object. More particularly, it relates to a projection optical device for use in the manufacture of semiconductor circuit devices.
In the semiconductor devices such as integrated circuits (IC), large scale integrated circuits (LSI) and very large scale integrated circuits (VLSI), circuit patterns have been radically miniaturized to achieve larger capacities, so that the minimum line width of the circuit patterns is required to be of the order of 1-2 microns. Various types of projection exposure apparatuses have been developed for the manufacture of these semiconductor devices, such as, for example, a reduction projection system, a one-to-one lens projection system or a mirror projection system. These projection exposure apparatuses are required to have a printing performance capable of printing a minute pattern of 1-2 microns line width and an alignment performance capable of achieving accurate alignment during various manufacturing steps, as well as stableness for minimizing the occurrence of wafer defects.
The printing performance of the projection exposure apparatus is determined by the effective F-number Fe of the projection optical system, for projecting the pattern of a mask onto a wafer, as well as the wavelength .lambda. of the printing beam, so that the depth of focus .DELTA.Z can be defined by: EQU .DELTA.Z=.+-.2.lambda.Fe.sup.2
In currently available reduction projection exposure apparatuses each having a printing performance of 1-micron line, a printing beam of 0.436-micron wavelength is used and an optical system having an effective F-number Fe of the order of 1.4 is employed. The depth of focus is thus of the order of .+-.1.7 microns. Therefore, the projection exposure apparatus must have a focus adjusting mechanism for positively imaging the photomask pattern on the wafer surface, and various adjusting mechanisms have been proposed up to now.
These focus adjusting mechanisms can be classified into two: one is a through-the-lens (TTL) system in which the wafer position is detected through a projection optical system and, on the basis of the detection results, the wafer is moved to the best imaging position; and another is a constant-distance system in which the wafer position is adjusted so that a constant distance is always maintained between the projection optical system and the wafer. The first-mentioned system requires a complicated optical system and causes various limitations to the design of the projection optical system. For this reason, most of the projection exposure apparatuses employ the focus adjusting mechanisms of the second-mentioned type.
In the already proposed focus adjusting mechanisms of the constant-distance type, the wafer position is detected by using air-nozzles, non-contact electric micrometers, optical sensors or the like. Each of these systems has a position detecting accuracy of the order of .+-.0.3 micron and ensures in co-operation with the adjusting mechanism the positioning of the wafer at a constant distance, relative to the projection optical system, satisfying the depth of focus.
The above-mentioned constant-distance system however involves inconveniences such as follows.
According to the constant-distance system, the wafer is always positioned at a constant distance from the projection optical system. Thus, in order that the wafer is always located at the best or optimum imaging position, the best imaging position of the projection optical system must be unchangeable.
When, however, the projection optical system is irradiated by the printing beam to effect printing of the wafer, the projection optical system absorbs a portion of the printing beam, as is known in the art, so that the optical performance changes due to the heat whereby the best imaging position is displaced.
In order to correct such changes, caused by the heat, in the optical performance of the projection optical system, it has been proposed that, with respect to the projection optical system, the focus error which is changeable with the lapse of the printing time is preparatively measured and one of the positions of the mask, the projection optical system and the wafer is corrected in accordance with the printing time. However, in addition to the heat caused by the printing beam, the projection optical system is affected by various heats, such as an ambient temperature, the heat which is generated within the interior of the projection exposure apparatus containing the projection optical system. Therefore, the correction of the changes in the optical performance is insufficient.
On the other hand, in order to prevent the effect of temperature with respect to the projection exposure apparatus, it has been proposed to use the projection exposure apparatus within a thermo-regulated room or to blow a thermo-regulated air over the entire projection exposure apparatus. Alternatively, it has been proposed to use a box-like member for housing the major part of the internal members of the projection exposure apparatus and to feed a fluid having a constant temperature into the box-like member.
However, each of these systems requires a bulky temperature control device, causing increase in the cost of manufacture. Moreover, it is difficult to completely prevent all the effects, relative to the projection optical system, of the heat caused by the light beam passing through the projection optical system, of the heat caused by the heat source contained within the interior of the projection exposure apparatus including the projection optical system, and of the changes in the ambient temperature or the like. It is therefore difficult to maintain the best or optimum optical performance.